This invention relates to a microwave landing system (MLS) for guiding aircraft to a safe landing at an airport and, more particularly, to a frequency-insensitive scanning-beam antenna which is adapted for inclusion within an array of airport landing lights by use of an offset phase center in an antenna excitation pattern.
Instrument landing systems, particularly microwave landing systems, are employed at airports for guiding aircraft safely to a landing on an airport runway during inclement weather, when visibility is restricted. Generally, an aircraft landing system employs both an electromagnetic guidance beam, transmitted frOm the round and received by the aircraft, and a series of approach lights, arranged typically in an ascending path from the end of the runway, to provide guidance on an inclined approach path, or glide path, which the aircraft is to follow during a landing procedure. The lights provide the pilot of an aircraft with a visual indication of the approach path, while the electromagnetic beam interacts with the receiver on board the aircraft to generate electrical signals which indicate the location of the aircraft relative to the desired approach path. The visual and electrical approach aids cooperate to ensure a safe landing. The electromagnetic beam enables the aircraft to follow the desired approach path at considerable distance from the runway, while the approach lights provide an additional visual aid and reference at relatively short distance from the runway during the final stage of aircraft approach.
Typically, in the construction of a modern MLS, a plurality of electromagnetic guidance beams are generated. One of these beams provides azimuth guidance by means of a vertical fan beam which is scanned to and from in azimuth to provide lateral guidance to orient the approaching aircraft. Of particular interest herein, is the construction of an azimuthal scanning antenna for generating such a horizontally scanned beam.
In a typical landing system, the series of approach lights is located between the near end of the runway and the approaching aircraft, while the guidance beam antenna is located beyond the far end of the runway. This arrangement of lights and antenna permits the electromagnetic beam to interact with the aircraft receiver even when the aircraft is flying above the runway immediately before touchdown. Both the guidance beam antenna and the set of approach lights are located along the runway axis.
However, to enable landings to be accomplished in either direction along a runway landing lights and guidance antennas are located at both ends of the runway. In this case, a problem arises in that the guidance-beam antenna used for guiding the aircraft in landing at one end of the runway must be located within the array of approach lights, and their supporting posts or towers, used in landing at the other end of the runway. As is well known, the supporting posts of the lights are typically relatively low near the end of the runway, so as to define a glide path for approaching aircraft. The placement of a guidance-beam antenna among the light-support posts is constrained by a restriction on the maximum height of the antenna. The antenna should not block any lights, nor be higher than an obstruction surface which is defined with respect to the light plane within the approach path. On the other hand, the presence of a nearby support post tends to perturb radiation from the antenna, particularly when the guidance beam is directed along the runway.
It has been the practice in the construction of a MLS to employ an array of columnar antenna element, arranged side-by-side, wherein each antenna element is a slotted waveguide with radiation emanating from a set of slot apertures on a front wall of each waveguide. Each waveguide supports a traveling wave, and is energized by a feed and phase shifter at the top of the waveguide. This exacerbates the foregoing problem by adding increased height to the antenna. Furthermore, the construction of the antenna with waveguide that employ traveling waves introduces a frequency dependence in the operation of the antenna, because the succession of slots along each waveguide functions as an array of slot radiators, introducing a predetermined elevation angle of radiation from each waveguide. The radiation beam squints in elevation as a function of frequency because the guide wavelength changes with frequency while the positions of the slots remain fixed. As a result, it has been necessary to mechanically tilt the array of columnar radiators to accomodate signal transmissions to the aircraft at different radiation frequencies to ensure that the radiated signals are directed in a desired elevation angle for reception by the aircraft. The frequency sensitivity, therefore, has necessitated undesirable increased complexity to the MLS antenna apparatus.
It is, therefore, an object of the present invention to provide new and improved antennas useful in aircraft landing systems.
It is another object of the present invention to provide new and improved aircraft landing systems wherein the guidance antennas are colocated with the guidance lights.
It is still another object of the present invention to provide new and improved waveguide radiator for use in an aircraft landing system, wherein said radiator employes a standing wave, a rear wall or bottom end feed and an offset phase center.